By the end of the year 2000, an estimated 36.1 million people worldwide were infected with HIV. In that year alone, HIV/AIDS-associated illnesses claimed the lives of approximately 3 million people worldwide. An estimated 500,000 of those deaths were of children under the age of fifteen. The importance of an HIV vaccine with respect to world health cannot be overstated.
It is recognized that effective vaccines that will inhibit or prevent HIV-1 infection or HIV-1 induced disease in humans will be useful for the treatment of certain high-risk populations, and as a general prophylactic vaccination for the general population that may risk HIV-1 infection or HIV-1 induced disease. A vaccine that will confer long-term protection against the transmission of HIV-1 would be most useful. Unfortunately, numerous problems stand in the way of developing effective vaccines for the prevention of HIV-1 infection and disease. Certain problems are most likely the result of the unique nature of the HIV-1 virus and its functional properties, and as yet no effective vaccine has been developed (for review, see: Berzofsky et al., Developing Synthetic Peptide Vaccines for HIV-1, Vaccines 95, pps. 135-142, 1995; Cease and Berzofsky, Toward a Vaccine for AIDS: The Emergence of ImmunobiologyBased Vaccine Design, Annual Review of Immunology, 12:923-989; Berzofsky, Progress Toward Artificial Vaccines for HIV, Vaccines 92, pps. 40-41, 1992).
HIV is a retrovirus, meaning that its genome consists of RNA rather than DNA. There are two primary strains of the virus, designated HIV-1 and HIV-2, with HIV-1 being the strain that is primarily responsible for human infection. The RNA genome of HIV is surrounded by a protein shell. The combination of RNA genome and the protein shell is known as the nucleocapsid, which is in turn surrounded by an envelope of both protein and lipid.
Infection of host cells by HIV begins when the gp120 protein of HIV, a highly glycosylated protein located in the viral envelope, binds to the CD4 receptor molecule of a host cell. This interaction initiates a series of events that allow fusion between the viral and cell membranes and the subsequent entry of the virus into the cell.
Following entry into the host cell, HIV RNA is transcribed into double-stranded DNA by a viral reverse transcriptase enzyme. Once integrated into the host genome, HIV expresses itself through transcription by the host's RNA Polymerase II enzyme. Through both transcriptional control and posttranscriptional transcript processing, HIV is able to exert a high level of control over the extent to which it expresses itself
Studies of the HIV virus have revealed much information about the molecular biology of the virus, including information concerning a number of genes and genetic regions important to the pathogenicity of HIV. Among these important genes and regions are rt, int, vif, and the 3′ LTR of HIV.
The rt gene of HIV encodes viral reverse transcriptase. This enzyme utilizes the RNA genome of HIV to produce a corresponding linear double-stranded DNA molecule that can be incorporated into the host genome.
The int gene of HIV encodes an integrase. This is the enzyme that actually catalyzes the insertion of the reverse-transcriptase-produced linear double-stranded viral DNA into the host genome. In order to complete integration of the viral DNA into the host genome, the host cell DNA repair machinery performs a ligation of the host and viral DNAs.
The vif gene of HIV encodes a protein known as the ‘viral infectivity factor.’ This protein is required for the production of infectious virions. The protein likely overcomes a cellular inhibitor that otherwise inhibits HIV-1, and may also enhance the stability of the viral core and the preintegration complex.
The LTR (Long Terminal Repeat) regions of HIV-1 contain promoter regions necessary to drive expression of the HIV genes. The 5′ LTR of HIV-1 contains the promoter that is primarily responsible for driving HIV-1 gene expression, though if the 5′ LTR sequence is disrupted, the 3′ LTR may assume this function. The 3′ LTR is necessary for integration of the viral DNA into the host genome.
Among other important HIV-1 genes are gag, pol, nef, and vpu.
The gag gene encodes for, among other things, the p27 capsid protein of HIV. This protein is important in the assembly of viral nucleocapsids. The p27 protein is also known to interact with the HIV cellular protein CyA, which is necessary for viral infectivity. Disruption of the interaction between p27 and CyA has been shown to inhibit viral replication.
The pol gene contains the rt and int sequences of HIV-1, thus encoding, among other things, reverse transcriptase and integrase.
The nef gene product (known as Negative Factor, or Nef) has a number of potentially important properties. Nef has the ability to downregulate CD4 and MHC Class I proteins, both of which are important to the body's ability to recognize virus-infected cells. Nef has also been shown to activate cellular protein kinases, thereby interfering with the signaling processes of the cell. Perhaps most importantly, deletion of nef from a pathogenic clone of Simian Immunodeficiency Virus (SIV) renders the virus nonpathogenic in adult macaque monkeys. Thus, a functional nef gene is crucial for the ability of SIV to cause disease in vivo. Further, studies have shown that HIV positive individuals with large deletions in the nef gene remained healthy for well over ten years, with no reduction in cellular CD4 counts.
The vpu gene encodes a protein of originally unknown function (known as Viral Protein, Unknown, or Vpu), but which is now known to downregulate CD4 and MHC Class-I expression as well as promote viral budding. Vpu is also similar to another viral protein that acts as an ion channel. The vpu gene is present in HIV-1, but is absent in HIV-2.
In nearly all viral infections, certain segments of the infected population recover and become immune to future viral infection by the same pathogen. Examples of typical viral pathogens include measles, poliomyelitis, chicken pox, hepatitis B, and small pox. The high mortality rate of HIV-1 infection, and the extremely rare incidence of recovery and protective immunity against HIV-1 infection, has cast doubt on the ability of primates to generate natural immunity to HIV-1 infection when pathogenic HIV-1 is the unmodified wild-type viral pathogen. Thus, there is a great need for a vaccine that will confer upon primate populations protective immunity against HIV-1 virus.
A hallmark of resistance to future viral infection is the generation of ‘neutralizing antibodies’ capable of recognizing the viral pathogen. Another measure is cellular immunity against infected cells. In typical viral infections, generation of neutralizing antibodies and cellular immunity heralds recovery from infection. In HIV-1 infection, however, neutralizing antibodies and cellular immunity appear very early during the infection and have been associated with only a transient decrease in viral burden. In spite of the generation of neutralizing antibodies and cellular immunity, viral replication in HIV-1 infection rebounds and AIDS (acquired immune deficiency syndrome) develops. Thus, in HIV-1 infection, neutralizing antibodies and cellular immunity are not accurate measures of protective immunity.
A further problem in developing an effective vaccine for HIV-1 is the antigenic diversity of the wild-type virus. There is a strong possibility that vaccines generated via recombinant HIV-1 coat proteins will confer resistance to specific phenotypes of virus and not broad spectrum immunity. Vaccine development using recombinant HIV-1 gp120 peptide, a HIV-1 viral coat protein, has passed phase-one clinical trials showing no toxicity. Data has indicated, however, that neutralizing antibodies appeared only transiently. Thus, recombinant HIV-1 gp120-peptide vaccines may act only in the short-term, with reversion to susceptibility of infection occurring in the future.
In general, it is accepted that live-virus vaccines induce better immunity against pathogenic viruses than isolated viral proteins (see, for example, Putkonen et al., Immunization with Live Attenuated SIVmac Can Protect Macaques Against Mucosal Infection with SIVsm Vaccines 96, pps. 200-210, 1996; Dimmock and Primrose Introduction to Modern Virology, Fourth Ed., Blackwell Science, 1994). The use of live lentivirus vaccines, such as HIV-1 vaccine, is resisted because of great concern that the virus will persist indefinitely in the inoculated population because of integration of the viral DNA into the host DNA of the inoculated individuals (see, for example, Haaft et al., Evidence of Circulating Pathogenic SIV Following Challenge of Macaques Vaccinated with Live Attenuated SIV, Vaccines 96, pps. 219224, 1996). Thus, a safe and effective vaccine against HIV-1 will encompass modifications to prevent the development of virulent pathogenic infection that could occur by either random mutation or other change to the initially non-pathogenic vaccine virus. One possibility for such a vaccine could come in the form of a DNA vaccine against HIV-1.
DNA vaccines are generally injected into host tissues in the form of plasmid DNA or RNA molecules via needle or particle bombardment. Once delivered, the DNA induces expression of antigenic proteins within transfected cells. U.S. Pat. No. 6,194,389 describes methods for transferring DNA to vertebrate cells to produce physiological immune-response producing protein in an animal subject and is incorporated herein in its entirety by reference.
Testing of vaccine efficacy generally requires the challenge of a subject with live virus or DNA. It is ethically and practically difficult to attempt preliminary studies using human subjects. The use of model systems for preliminary design and testing of candidate vaccines has been hampered by various species-specific features of the virus. The HIV-1 virus itself is currently known only to infect certain rate and endangered species of chimpanzees in addition to humans. The feasibility of obtaining sufficient numbers of such endangered animals for full preliminary study of HIV-1 virus vaccines is quite low. It is preferable to use validated analogous animal model systems.
One analogous model system for HIV-1 has been the SIVmac (Simian Immunodeficiency Virus, macaque) system. SIV infects a variety of simians, including macaques, but the differences between SIV and HIV make SIV of limited use as a potential human vaccine. There is, therefore, a need for a vaccine made from a virus that is closely related to HIV, but still infectious in an animal model for purposes of testing.
Chimeric SIV-HIV virus has been developed by placing the envelope proteins of HIV-1 on a background of SIVmac. The chimeric virus proved to be infectious in monkeys, but did not result in full-blown AIDS or an accurate model to mimic HIV-1 infection monkeys.
As described below, the present invention teaches specific DNA constructs and methods that are effective in generating an immune response to HIV-1 in a vaccinated host.